Problem: $ { {3} \times \left[\begin{array}{rr}{4} & {3} \\ {-1} & {1}\end{array}\right] = {?} } $
Solution: To multiply a matrix by a number, multiply each item of the matrix by the number. ${\left[\begin{array}{rr}{3}\times{4} & {3}\times{3} \\ {3}\times{-1} & {3}\times{1}\end{array}\right]}$ Now, simplify: ${\left[\begin{array}{rr}{12} & {9} \\ {-3} & {3}\end{array}\right]}$